#include<bits/stdc++.h>
using namespace std;

int Fib(int a){
    if(a<=1){
        return 1;
    }else{
        return Fib(a-1)+Fib(a-2);//分治法
    }
}

//提出对分支法进行改进
long Fib(long x){
    long x1=0,x2=1,sum;
    for(int i=2;i<=x;i++){
        sum = x1+x2;
        x1 = x2;x2 = sum;
    }
    return sum;
}